LOFAR, the LOw-Frequency ARray, is a new-generation radio interferometer constructed in the north of the Netherlands and across europe. Utilizing a novel phased-array design, LOFAR covers the largely unexplored low-frequency range from 10-240 MHz and provides a number of unique observing capabilities. Spreading out from a core located near the village of Exloo in the northeast of the Netherlands, a total of 40 LOFAR stations are nearing completion. A further five stations have been deployed throughout Germany, and one station has been built in each of France, Sweden, and the UK. Digital beam-forming techniques make the LOFAR system agile and allow for rapid repointing of the telescope as well as the potential for multiple simultaneous observations. With its dense core array and long interferometric baselines, LOFAR achieves unparalleled sensitivity and angular resolution in the low-frequency radio regime. The LOFAR facilities are jointly operated by the International LOFAR Telescope (ILT) foundation, as an observatory open to the global astronomical community. LOFAR is one of the first radio observatories to feature automated processing pipelines to deliver fully calibrated science products to its user community. LOFAR's new capabilities, techniques and modus operandi make it an important pathfinder for the Square Kilometre Array (SKA). We give an overview of the LOFAR instrument, its major hardware and software components, and the core science objectives that have driven its design. In addition, we present a selection of new results from the commissioning phase of this new radio observatory.

Future high-redshift 21-cm experiments will suffer from a high degree of contamination, due both to astrophysical foregrounds and to non-astrophysical and instrumental effects. In order to reliably extract the cosmological signal from the observed data, it is essential to understand very well all data components and their influence on the extracted signal. Here we present simulated astrophysical foregrounds data cubes and discuss their possible statistical effects on the data. The foreground maps are produced assuming 5 • × 5 • windows that match those expected to be observed by the LOFAR epoch of reionization (EoR) key science project. We show that with the expected LOFAR-EoR sky and receiver noise levels, which amount to ≈52 mK at 150 MHz after 400 h of total observing time, a simple polynomial fit allows a statistical reconstruction of the signal. We also show that the polynomial fitting will work for maps with realistic yet idealized instrument response, i.e. a response that includes only a uniform uv coverage as a function of frequency and ignores many other uncertainties. Polarized Galactic synchrotron maps that include internal polarization and a number of Faraday screens along the line of sight are also simulated. The importance of these stems from the fact that the LOFAR instrument, in common with all current interferometric EoR experiments, has an instrumentally polarized response. 5 We assume a Lambda cold dark matter ( CDM) universe with b = 0.04, m = 0.26, = 0.738 and H 0 = 70.8 k ms −1 Mpc −1 .

We present the first limits on the Epoch of Reionization 21 cm H I power spectra, in the redshift range z=7.9-10.6, using the Low-Frequency Array (LOFAR) High-Band Antenna (HBA). In total, 13.0 hr of data were used from observations centered on the North Celestial Pole. After subtraction of the sky model and the noise bias, we detect a non-zero 56 13 mK D < ( ) at k=0.053 h cMpc −1 in the range z=9.6-10.6. The excess variance decreases when optimizing the smoothness of the direction-and frequency-dependent gain calibration, and with increasing the completeness of the sky model. It is likely caused by (i) residual side-lobe noise on calibration baselines, (ii) leverage due to nonlinear effects, (iii) noise and ionosphere-induced gain errors, or a combination thereof. Further analyses of the excess variance will be discussed in forthcoming publications.

The Square Kilometre Array (SKA) will have a low frequency component (SKA-low) which has as one of its main science goals the study of the redshifted 21cm line from the earliest phases of star and galaxy formation in the Universe. This 21cm signal provides a new and unique window both on the time of the formation of the first stars and accreting black holes and the subsequent period of substantial ionization of the intergalactic medium. The signal will teach us fundamental new things about the earliest phases of structure formation, cosmology and even has the potential to lead to the discovery of new physical phenomena. Here we present a white paper with an Executive SummaryThe Square Kilometre Array (SKA) will have a low frequency component (AA-low/SKA-low 1 ) which has as one of its main science goals the study of the redshifted 21cm line from the earliest phases of star and galaxy formation in the Universe (see SKA Memo 125). It is during this phase that the first building blocks of the galaxies that we see around us today, including our own Milky Way, were formed. It is a crucial period for understanding the history of the Universe and one for which we have currently very little observational data.We divide the period into two different phases based on the physical processes which affect the Intergalactic Medium. The first period, which we call the Cosmic Dawn, saw the formation of the first stars and accreting black holes, which changed the quantum state of the still neutral Intergalactic Medium. The second period, known as the Epoch of Reionization, is the one during which large areas between the galaxies were photo-ionized by the radiation produced in galaxies and which ended when the Intergalactic Medium had become completely ionized.Observations of the redshifted 21-cm line with SKA will provide a new and unique window on the entire period of Cosmic Dawn and Reionization. The signal is sensitive to the emergence of the first stellar populations, radiation from growing massive black holes and the formation of larger groups of galaxies and bright quasars. At the same time it maps the distribution of most of the baryonic matter in the Universe. The study of the redshifted 21cm line will teach us fundamental new things about the earliest phases of structure formation and cosmology. It even has the potential to lead to the discovery of new physical phenomena. Here we present an overview of the science questions that SKA-low can address, how we plan to tackle these questions and what this implies for the basic design of the telescope.The redshifted 21cm signal will be analyzed with different techniques, which each come with their own requirements for the SKA: (i) Tomography, (ii) power-spectra and higher-order statistics, (iii) hydrogen absorption, (iv) global/total-intensity signal. Whereas all precursors/pathfinders aim to study the signal statistically through its power spectrum, SKA will be able to image the neutral hydrogen distribution directly and its focus will therefore be more on tomograph...

A new upper limit on the 21-cm signal power spectrum at a redshift of z ≈ 9.1 is presented, based on 141 hours of data obtained with the Low-Frequency Array (LOFAR). The analysis includes significant improvements in spectrally-smooth gain-calibration, Gaussian Process Regression (GPR) foreground mitigation and optimally-weighted power spectrum inference. Previously seen 'excess power' due to spectral structure in the gain solutions has markedly reduced but some excess power still remains with a spectral correlation distinct from thermal noise. This excess has a spectral coherence scale of 0.25 − 0.45 MHz and is partially correlated between nights, especially in the foreground wedge region. The correlation is stronger between nights covering similar local sidereal times. A best 2-σ upper limit of ∆ 2 21 < (73) 2 mK 2 at k = 0.075 h cMpc −1 is found, an improvement by a factor ≈ 8 in power compared to the previously reported upper limit. The remaining excess power could be due to residual foreground emission from sources or diffuse emission far away from the phase centre, polarization leakage, chromatic calibration errors, ionosphere, or low-level radio-frequency interference. We discuss future improvements to the signal processing chain that can further reduce or even eliminate these causes of excess power.

Context. The cosmological 21 cm line promises to be a formidable tool for cosmology, allowing the investigation of the end of the so-called dark ages, when the first galaxies formed. Aims. Astrophysical foregrounds are expected to be about three orders of magnitude greater than the cosmological signal and therefore represent a serious contamination of the cosmological 21 cm line. Detailed knowledge of both their intensity and polarization structure on the relevant angular scale of 1-30 arcmin will be essential for extracting the cosmological signal from the data. Methods. We present the first results from a series of observations conducted with the Westerbork telescope in the 140-160 MHz range with a 2 arcmin resolution aimed at characterizing the properties of the foregrounds for epoch of reionization experiments. The polarization data were analysed through the rotation measure synthesis technique. We computed total intensity and polarization angular power spectra. Results. For the first time we have detected fluctuations in the Galactic diffuse emission on scales greater than 13 arcmin at 150 MHz, in the low Galactic latitude area known as Fan region, centred at α = 3 h 10 m , δ = 65 • 30 . Those fluctuations have an rms of 14 K. The total intensity power spectrum shows a power-law behaviour down to ∼900 with slope β I = −2.2 ± 0.3. The detection of diffuse emission at smaller angular scales is limited by residual point sources. We measured an rms confusion noise of ∼3 mJy beam −1 . Diffuse polarized emission was also detected for the first time at this frequency. The polarized signal shows complex structure both spatially and along the line of sight. The polarization power spectrum is not affected by residual point sources and is only limited by the thermal noise. It shows a power-law behaviour down to ∼2700 with slope β P = −1.65±0.15. The rms of polarization fluctuations is 7.2 K on 4 arcmin scales. Conclusions. The measured total intensity fluctuations are used to estimate the foreground contamination on the cosmological signal.By extrapolating the spectrum of total intensity emission, we find a contamination of δT = ( + 1)C I /2π ∼ 5.7 K on 5 arcmin scales and a corresponding rms value of ∼18.3 K at the same angular scale. The level of the polarization power spectrum is δT ∼ 3.3 K on 5 arcmin scales. However, the Fan region cannot be taken as representative of other sky regions, given its exceptionally bright polarized signal, but is likely to represent an upper limit on the sky brightness at moderate and high Galactic latitude.

Aims. This study aims to characterize linear polarization structures in LOFAR observations of the interstellar medium (ISM) in the 3C 196 field, one of the primary fields of the LOFAR-Epoch of Reionization key science project. Methods. We have used the high band antennas (HBA) of LOFAR to image this region and rotation measure (RM) synthesis to unravel the distribution of polarized structures in Faraday depth. Results. The brightness temperature of the detected Galactic emission is 5−15 K in polarized intensity and covers the range from -3 to +8 rad m −2 in Faraday depth. The most interesting morphological feature is a strikingly straight filament at a Faraday depth of +0.5 rad m −2 running from north to south, right through the centre of the field and parallel to the Galactic plane. There is also an interesting system of linear depolarization canals conspicuous in an image showing the peaks of Faraday spectra. We used the Westerbork Synthesis Radio Telescope (WSRT) at 350 MHz to image the same region. For the first time, we see some common morphology in the RM cubes made at 150 and 350 MHz. There is no indication of diffuse emission in total intensity in the interferometric data, in line with results at higher frequencies and previous LOFAR observations. Based on our results, we determined physical parameters of the ISM and proposed a simple model that may explain the observed distribution of the intervening magneto-ionic medium. Conclusions. The mean line-of-sight magnetic field component, B , is determined to be 0.3 ± 0.1 µG and its spatial variation across the 3C 196 field is 0.1 µG. The filamentary structure is probably an ionized filament in the ISM, located somewhere within the Local Bubble. This filamentary structure shows an excess in thermal electron density (n e B > 6.2 cm −3 µG) compared to its surroundings.

The low frequency array (LOFAR), is the first radio telescope designed with the capability to measure radio emission from cosmic-ray induced air showers in parallel with interferometric observations. In the first ∼2 years of observing, 405 cosmic-ray events in the energy range of 10 16 −10 18 eV have been detected in the band from 30−80 MHz. Each of these air showers is registered with up to ∼1000 independent antennas resulting in measurements of the radio emission with unprecedented detail. This article describes the dataset, as well as the analysis pipeline, and serves as a reference for future papers based on these data. All steps necessary to achieve a full reconstruction of the electric field at every antenna position are explained, including removal of radio frequency interference, correcting for the antenna response and identification of the pulsed signal.